This invention relates to a method for the automatic control of machining process, in particular the cutting by means of a machine tool of a series of workpieces having surfaces of different characteristics to be machined by variation of the feed speed of a cutting tool of the machine tool from workpiece to workpiece.
More particularly the invention concerns processes of said type which make use of variables which affect the cost of the workpiece in adapting automatically the feed speed of the cutting tool to prevailing conditions, including the cutting efficiency of the tool, variation of the depth of cut, variation of the area of the surface being machined and of the hardness of the workpiece material.
The main object of the invention is to provide a process for controlling a machine tool to allow the optimum use of the tool, taking into account, in addition to the abovementioned variables, specific economic considerations; in particular the invention seeks to provide a method in which an increase in the depth of cut within the working limits of the machine, along a workpiece surface which is being machined does not lead to an increase in the machining time.
Processes are known for the control of production machining, by means of machine tools, of a series of workpieces in which the temperature of the cutting region is measured and in which the state of wear of the cutting tool is also monitored constantly, these two parameters being utilized in the automatic control of the cutting speed of the tool. Such known processes serve to reduce the cutting speed with time so that the temperature of the cutting region remains substantially constant and the tool wear does not exceed predetermined limits.
The fundamental disadvantage of such processes lies in the fact that the measurement of the temperature in the cutting region is particularly complicated and difficult. In fact, this temperature is influenced by the mass flow of coolant, the form of cuttings produced and by variations in the conditions of contact between the cutting tool and the workpiece, which in turn depend on the type of tool and which vary with each sharpening of the tool. The temperature in the cutting region therefore undergoes considerable variations even over very short periods, and in practice the temperature measurements will be limited to the mean temperature over a period and the mean temperature of an area of the workpiece of excessive dimensions.
Another known process for controlling the operation of a machine tool entails measuring the working torque applied by a cutting tool to the workpiece, the power applied to the tool and the deflection to which the tool is subjected during working. In such a process the quantities which are controlled are the cutting speed and the advance feed speed of the tool. The controlled quantities are tied to quantities measured by rather complicated predetermined equations. Control of the cutting speed and of the feed speed of the tool is effected by an analog computer which solves the said equations. This process suffers from fundamental disadvantage of using electronic apparatus and specially programmed measuring instruments which are both very complex and costly.